289 FXUS63 KGRR 071730 AFDGRR Area Forecast Discussion National Weather Service Grand Rapids MI 1230 PM EST Sun Nov 7 2021 LATEST UPDATE... Aviation .SYNOPSIS... Issued at 329 AM EST Sun Nov 7 2021 - Dry through Monday Night at least - Small chance for rain on Tuesday - Deep fall system to our west mid week - Turning colder with lake effect possible late in the week && .DISCUSSION...(Today through next Saturday) Issued at 329 AM EST Sun Nov 7 2021 - Dry through Monday Night at least We will be dry through Monday night at least, with the potential to be dry through Tuesday night. More on the chance for rain on Tuesday in the paragraph below. Ridging aloft today and tonight transitions into zonal flow for Monday and Monday night. Without the lack up upper support see no chance for precipitation through Monday night. Deeper moisture holds off until later into Monday night, so a high confidence that we remain dry through Monday night. Hard to find much nicer days in November than today and tomorrow as highs will be up around 60F both days. If you have yard work to finish up before we head too far into winter, the next two days will be great for that. - Small chance for rain on Tuesday We have a small chance for rain on Tuesday but there is model disparity here. The GFS is dry and its ensemble members support this. The ECMWF has some light rain and its ensemble members support line of thinking. The precipitation with the ECMWF is associated with a weak mid level wave and surface reflection riding up a boundary that will be stalled across the area. If the ECMWF verifies the rain will be light, on the order of a tenth of an inch or less in most places. At this point its a tough call as to which model verifies and we have given a nod to the ECMWF with some low pops (20-30 pct) on Tuesday. - Deep fall system to our west mid week The ECMWF has now joined the GFS in showing a deep fall storm system off to our west in the Wednesday/Thursday time frame. Last night the ECMWF had a much weaker low. Now, both models are showing a 983mb low on Thursday. This is obviously quite deep, but is something that is in the realm of what can occur this time of year in the Great Lakes. In fact, all of the biggest fall storms, weirdly fall in and around Nov 10-11th. This one would be no different. There has been some hint in the max wind gusts at places like Ludington of 50+ mph winds during this time frame which is certainly possible given the depth of the low. So, a windy and wet system is expected in our area Wednesday and Thursday. We would see a warm air advection wing of precipitation on Wednesday, followed by cold front and another round of rain Wednesday night into Thursday. We look to be dry slotted for a time Thursday night. The jury is still out on how this all evolves though as disparity in the operational runs show up Thursday night. The ECMWF Thursday night has a much weaker low in Iowa which doesn't seem reasonable. Bottom line, some impactful weather with this system, especially in regard to some wind gusts along the Lake Michigan shore. - Turning colder with lake effect possible late in the week We still look to turn colder late in the week behind the fall storm. That said, the ECWMF has backed off on the cold air advection due to a weaker and further southwest low. Friday night there is a 6 degree difference in 850mb temps (-7C vs -1C) near Muskegon at 850mb. This could be the difference in healthy lake effect precipitation or not. The lake is around +12C right now, so if the GFS verifies we are looking at Delta Ts approaching 20C. Thinking the GFS is likely the way to go here given colder and lake effect rain/snow would be common behind a deep fall storm system. Plenty of time to watch how this unfolds in the models though. Can envision some accumulating lake effect snow in Western Lower Michigan into next weekend given deep moisture, an upper trough overhead and plenty of instability (at least in the GFS). && .AVIATION...(For the 18Z TAFS through 18Z Monday afternoon) Issued at 1230 PM EST Sun Nov 7 2021 It will remain VFR with scattered cirrus clouds and winds from the southwest. MKG will tend to stay gusty overnight due to proximity to Lake Michigan. At other locations, ground level winds will weaken late tonight while winds in the 1000 to 2000 ft AGL layer increase. Low level wind shear greater than 30 knots between the ground and 1500 feet is likely in the early morning hours until daybreak. && .MARINE... Issued at 329 AM EST Sun Nov 7 2021 Winds and waves will trend down the remainder of the early morning hours. Will let the Small Craft Advisory play out/expire through 700am. At last check the mid lake buoys were indicating 6 foot waves on the north portion of the lake and 4 foot waves to the south. So, the current SCA looks good. We have another increase in south winds tonight, so another SCA will be needed for that time frame. We considered bridging the gap and just running the SCA through today as we will likely see 2-4 foot waves all day. In the end we decided to keep this two separate events. So, look for another SCA for tonight coming up. Its conceivable that we may need one for today as well if the winds/waves overperform which is certainly possible. The new SCA will be needed for all zones for tonight and Monday. && .GRR WATCHES/WARNINGS/ADVISORIES... MI...None. LM...Small Craft Advisory from 7 PM this evening to 7 PM EST Monday for LMZ844>849. && $$ SYNOPSIS...Duke DISCUSSION...Duke AVIATION...CAS MARINE...Duke